Validity and contributions to pain severity from the Central Aspects of Pain (CAP) questionnaire in rheumatoid arthritis

Background: The central nervous system (CNS) contributes to pain perception across musculoskeletal conditions. The Central Aspects of Pain (CAP) questionnaire captures a single score associated with quantitative sensory testing (QST) evidence of CNS dysfunction validated in knee osteoarthritis. Given the different pathophysiology of rheumatoid arthritis (RA), an inflammatory polyarthritis this cross-sectional study assessed CAP’s psychometric properties and its association with pain in RA. Methods: Adults with RA were recruited from Nottinghamshire, London and Cardiff. Participants completed CAP and reported pain using a numerical rating scale. A subgroup underwent additional assessments, including quantitative sensory testing (QST; Pressure Pain detection Threshold, Temporal Summation, Conditioned Pain Modulation), Disease Activity Score-28, C-reactive protein, questionnaires addressing pain and related characteristics, and Central Sensitization Inventory short form (CSI-9). Cronbach alpha, confirmatory factor (CFA) and Rasch measurement theory (RMT) assessed CAP’s reliability and validity. Multivariable linear regression modelled contributions to pain by inflammation indices and CAP or CSI-9. Results: The 380 participants (73% female, median 63y) reported average pain over the past four weeks of 6/10 and a CAP score of 9/16. CAP demonstrated acceptable reliability (ICC(2,1)=0.71), CFA fit (CFI=0.99, TLI=0.99, RMSEA=0.034, SRMR=0.03), and internal consistency (α=0.82). CAP was significantly associated with pain (0.50≤β≤0.57) but not QST. CAP explained 33% of pain variance, rising to 42% with inflammation, age, sex, and BMI. CSI-9 correlated with pain, not QST and explained less pain variance than CAP. Conclusion: CAP is reliable and valid for use with people with RA and explains RA pain variance better than inflammation or CSI-9.